Hydrogen (H2) is used in many different applications, such as the synthesis of various substances, reduction, hydrodesulfurization of petroleum, and hydrogenolysis, and is needed in every industrial field. For example, fuel cells, which have been attracting attention in recent years, are capable of supplying electricity continuously and efficiently when reactants such as hydrogen and oxygen are supplied externally thereto. Research on the fuel (reactant) of practical fuel cells mainly has focused on the use of methanol. However, when methanol is burned, formation of a poisoned by-product, for example, incompletely oxidized substances such as carbon monoxide and hydrocarbons, on the surface of an electrode catalyst is a problem. Thus, it is desirable to supply hydrogen, which is a clean fuel, to fuel cell electrodes. For the foregoing reasons, hydrogen supply or storage techniques are industrially very important. However, the stable supply or storage of hydrogen has been difficult so far because hydrogen is a gas at room temperature, has high reactivity and thus readily ignites in air, and so on.
For example, a method for storing hydrogen as a compressed gas is commonly used as a hydrogen storage method. However, this method is costly because it is necessary to overcome problems such as the safe transportation of compressed gas, hydrogen brittleness of container materials, and the like. Moreover, another hydrogen storage method is a method of storing hydrogen in the form of liquid hydrogen by liquefying hydrogen gas. However, this method has problems in that a great deal of energy is required in the step of liquefying hydrogen gas, a special and expensive container is required to store liquefied hydrogen, and so on. Still another hydrogen storage method is a method of storing hydrogen using a hydrogen storage alloy that absorbs hydrogen. However, hydrogen storage alloys have problems in that the repeated storage and release of hydrogen result in pulverization, and the pulverized hydrogen storage alloys are likely to suffer performance degradation; the hydrogen storage alloys are heavy, for example, and, therefore, are difficult to handle; a large amount of heat is produced and absorbed when the hydrogen storage alloys absorb and desorb hydrogen; and so on.
A possible method to solve these problems is a method of storing hydrogen in the form of a substance other than H2. For example, formic acid (HCOOH) is known to generate hydrogen (H2) and carbon dioxide (CO2) when strongly heated. This property can be used to store hydrogen in the form of formic acid, which is a safe substance, and generate hydrogen by heating formic acid at appropriate temperature, thereby supplying hydrogen sustainably. It can be said that since formic acid is naturally available and also can be produced biologically, formic acid is effective as an environmentally-friendly hydrogen source that does not use fossil fuels. However, thermal decomposition of formic acid by simply heating formic acid has problems of cost and the like because a high temperature higher than the boiling point (101° C.) of formic acid and the melting point (253° C.) of sodium formate is required. Therefore, the development of a catalyst that is capable of efficiently generating hydrogen from formic acid under mild conditions has been sought.
Various catalysts for the decomposition of formic acid in which a metal complex is used previously have been researched (Non-Patent Documents 1 to 4 and the like), but those catalysts had problems in terms of the reactivity and the like of the catalysts, especially with the application of the catalysts to fuel cells. On the other hand, research on catalysts for the decomposition of formic acid that are solid catalysts recently has been conducted extensively in order to put-formic acid fuel cells to practical use. For example, Tekion, Inc., an affiliate of BASF, placed a formic acid fuel cell for mobile computers on the market for the first time in 2006 (Non-Patent Documents 5 and 6). However, these solid catalysts are expensive because high-priced precious metals, such as platinum, palladium, their alloys, or the like, are used (see Non-Patent Document 7, for example).
Non-Patent Document 1: Ford, P. C. et al., J. Am. Chem. Soc., 1977, 99, 252
Non-Patent Document 2: Otsuka, S. et al., J. Am. Chem. Soc., 1978, 100, 3941
Non-Patent Document 3: Lau, C. P. et al., Dalton, 2003, 3727
Non-Patent Document 4: Puddephatt, R. J. et al., Dalton, 2000, 3212; Chem. Commun., 1998, 2365
Non-Patent Document 5: [online] Mar. 13, 2006, BASF, [retrieved on Nov. 6, 2006], from the Internet <URL: http://www.corporate.basf.com/en/presse/mitteilungen/pm.htm?pmid=2188&id=V00-PCnAH9TaSbcp2Hn>
Non-Patent Document 6: [online], 2006, Tekion, Inc., [retrieved on Nov. 6, 2006], from the Internet <URL: http://www.tekion.com/main.htm>
Non-Patent Document 7: Masel, R. I. et al., Fuel Cells, 2004, 4, 337